I can sympathize with what you're going through. I, too, share your frustration. First off, don't feel alienated or like some Luddite under Snow Leopard. There are many of us who still run that version on their Macs, either for the same reason of disliking Lion and later or because the hardware limits our ability to upgrade. For me, it's the latter.
My dad owns a Mac Book Pro Mid-2010 (MacBook Pro 6,2; Unibody, non-Retina), top-of-the-line at the time. It was intended for use when he was still working in fire prevention. It served him well, until politics caused him to retire early and the laptop was given to him as a going away present. The department didn't know how to use it and how to integrate it into the world of their vast network of Windows PCs, but that's another story.
Anyway, things were fine under Snow Leopard (stock version that came with the MBP). When it came time to upgrade to Lion, we did so, but began having random crashes. Thinking this was OS X, we waited for new updates to try and see if it would fix it. Lion's updates weren't promising anything. When Mountain Lion came out, we tried again, with no luck; it just got worse. Returning back to Snow Leopard was far stable and the only tolerable solution.
Doing a bit of research, it appeared that this was actually a hardware-related issue. On this model, it is equipped with 2 graphics cards: a lower powered integrated Intel HD Graphics and an Nvidia GeForce GT 330 M. When you are doing light amounts of graphics work (loading OS X, using iWork, etc.), the Intel HD Graphics is utilized. But if you are doing more aggressive graphics tasks (using Flash Player under Firefox, plugging in an external display, playing games, etc.), the Nvidia is used. OS X has an included feature called Automatic Graphics Switching starting in Snow Leopard and later. So far, this feature holds up well under Snow Leopard, but has major issues on the later OS X versions. Someone was even gracious enough to create an app called gfxCardStatus to put the switching under manual control, but it too doesn't fix the issue; it just allows us to minimize how often it happens. There are some times where the MBP will go black screen suddenly with no return to the state it was in (under Snow Leopard); OS X is running, but I can't get the MBP to respond. The solution for us is to manually power cycle with the power button and all is well.
Neither Apple nor Nvidia has acknowledged this issue, despite numerous customers being in the same boat and we've all sent them crash reports. Sadly for us, the 3 year AppleCare warranty is long used up and will cost us hundreds of dollars to get Apple to fix it, without any guarantee that a replacement MBP logic board will permanently fix the problem. Now, 5 years after first buying the MBP, we are now very leery of ever buying Apple products again.
I don't want to scare people away from Apple, but I feel most of us are screwed and left holding the bag. This would never have happened if Steve Jobs were still alive; he would've at least start asking questions and done something to help us. But Steve is no longer with us and the company still rolls on in their future pursuits. Fine. I wish them well.
I can't say much for the GUI or its features, because I don't have a working Mac to enjoy learning it on and I don't have the money to go buy a new one. Added the fact, if quality control of their products are this bad, why would I bother to buy another lemon anyway? I'm pretty sure there are many improvements over earlier OS X versions, but I doubt I would use all of it.
Regarding the PowerMac G4 (QuickSilver), you didn't mention which of the 3 versions you have. According to the mighty MacTracker, the earliest version is code named Titan or Nichrome and lifespan from July 2001-January 2002. The second version (QuickSilver 2002) has no code name, but lifespan from January 2002-August 2002. The third version (QuickSilver 2002ED), again no code name, but lifespan from August 2002 until the release of the PMG4 Mirror Drive Door.
The first edition gave you a choice of single 733 Mhz, single 867 Mhz or dual 800 Mhz CPUs with a 4x AGP Nvidia GeForce2 MX, same card w/ TwinView or GeForce3. The second version gave you a choice of single 733, single 800, single 933 or dual 1.0 Ghz CPUs with a 4x AGP ATI Radeon 7500, an Nvidia GeForce4 MX or GeForce4 Ti. The third version gave you a single 867 Mhz CPU with a 4x AGP Nvidia GeForce4 MX.
Whichever video card you decide to upgrade to will need to be of 4x AGP standard either PC or Mac Edition. You may also need an adapter to go from DVI female to ADC, unless the card is equipped with one. I would check eBay and Amazon (I prefer Amazon myself) and see what's available. Low End Mac provided this link deep in its bowels which may be useful for you.
As for the PowerBook 1400, I couldn't find anyone on eBay or through some obvious Mac specialist companies who had those PRAM batteries in stock. You may however want to try these guys. They listed a possible match at between $20-$30 dollars. One note: they're in the UK, I'm not sure how much the shipping would be, plus they're currently out of stock. But you could have them notify you as soon as they get more in. Another possible alternative would be to see if an "expert" can build you a pram battery pack utilizing Nickel Metal Hydride (Ni-MH) button cells. It appears to be a VARTA battery pack; 7.2 Volt, 70 mAh (milliamp hours) or a 6 Volt, 80 H (Hour) rating(s). The "expert" will need to take apart the old pram battery and try to match part-for-part, connection-by-connection (including the wire pigtail that extends out the pack), so that there's no risk of something going horribly wrong, such as shorting out or catching fire. Done the wrong way can pose these risks.
I hope everything I've said is helpful. Hang in there; we're all in this together.